1. Field of the Invention
The present invention relates to a multiply-tuned probe for magnetic resonance imaging or spectroscopy, and more particularly, to a probe in which the driving inductor is coupled to two or more trap inductors which can be independently adjusted to separate resonant frequencies.
2. Description of the Prior Art
In multinuclear nuclear magnetic resonance (NMR) spectroscopy, a sample coil and associated tuning circuit (together commonly called the probe) must be resonant at the Larmor frequency of each nucleus under observation. Multi-nuclear NMR allows separate physiological experiments to be done simultaneously on a single sample, thereby ensuring good correlation between disparate measurements. However, the ability to concurrently acquire NMR signals from multiple nuclei depends upon having a probe which can be multiply-tuned to the respective frequencies for each nuclei. The sensitivity at each frequency must be good enough to allow adequate signal to noise levels, and ideally, the signal to noise ratio should be comparable at each frequency with a probe singly-tuned to that frequency. Optimum signal to noise ratios of each nucleus have typically been achieved by matching the impedance of the probe at each resonant frequency to the nominal impedance of the spectrometer. Furthermore, the impedance at all resonant frequencies of a multiply-tuned probe must be simultaneously matched to the impedance of the rest of the transmit/receive circuitry for the most efficient transmission of power. However, impedance matching for all nuclei has not been satisfactorily achieved in a repeatable manner.
Despite the above-mentioned problems with impedance matching, the use of multiply-tuned probes for in vivo multinuclear NMR studies is now commonplace. However, while multiply-tuned probes can be quite efficient when compared to singly-tuned probes, it remains difficult to match to the impedance of the spectrometer at multiple frequencies. For example, when probes are multiply-tuned without overcoupling, a large interaction among the respective coils makes it difficult to tune and impedance match the probe to the transmit/receive circuitry. As a result, the coils of prior art multiply-tuned probes have had to be tuned separately to the desired resonant frequencies and then tuned together so as to compensate for the effect each coil has on each other coil. It is desirable for each frequency to be as independently tunable as possible so as to make simultaneous optimization of all coil parameters possible.
FIG. 1 illustrates a conventional single-tuned surface coil probe having a sample inductor L.sub.s which is transformer coupled to a primary coil L.sub.p. The sample inductor L.sub.s and variable capacitance C.sub.s form an LC circuit which may be tuned to a desired resonant frequency by varying the capacitance of variable capacitor C.sub.s. A multiply-tuned probe is considerably more sophisticated than the probe of FIG. 1 in that a multiply-tuned probe may be tuned to two or more desired resonant frequencies concurrently such that separate physiological experiments may be done simultaneously on a single sample. However, at present, because of the difficulty in matching the probe to the impedance of the spectrometer at multiple frequencies, prior art multiply-tuned probes generally have been limited to double-tuned probes or surface coils for obtaining proton images and localized spectra of other nuclei from the same region of interest where the signal to noise ratio is optimized for only one frequency.
Over the past several years, a wide variety of such double-tuned surface coils have been disclosed. Ideally, such double-tuned surface coils provide high signal to noise ratios at each resonant frequency of interest, provide high homogeneity of the RF field B.sub.1 and allow operation on multiple frequencies without retuning or cable changes. However, in practice, prior art double-tuned surface coils have traded off between an acceptable level of B.sub.1 homogeneity and an acceptable signal to noise ratio.
Double-tuned surface coils of the prior art have been grouped according to the mechanism by which they achieve double resonance by Fitzsimmons et al. in an article entitled "A Comparison of Double-Tuned Surface Coils", Magnetic Resonance in Medicine, Vol. 10, pp. 302-309 (1989). Fitzsimmons et al. therein compare the operation of existing double-tuned surface coil designs and note that the oldest approach for achieving double resonance is to use quarter wave transmission lines to tune and match. Such designs incur a loss which is due to the fact that a quarter wave line always has a finite impedance even when one end is open circuited. The "short circuit" loss appears in series with a tuned circuit and produces a loss in circuit Q. Fitzsimmons et al. also recognize that the variable length lines have been used to place tuning components outside of the sample environment so as to cause circulating currents from the tank circuit to pass through the transmission line, resulting in reduced efficiency. Since the transmission line is not being utilized at its characteristic impedance, the resulting losses are a function of the line equivalent resistance in series with the resistance of the tuned circuit. Because high Q circuits have very low series resistance, the resistance added by the transmission lines significantly degrades the Q. For these reasons, prior art arrangements typically match the impedance of the tuned circuit to the impedance of the transmission line in an attempt to improve efficiency.
In order to reduce the losses inherent in transmission line schemes, others have proposed designs which use LC networks and simulated quarter wave lines. These approaches result in currents circulating in inductors (traps) which are not coupled to the sample. Hyde et al. introduced a double-tuned version of the loop gap resonator wherein the coil utilizes a pair of loop gap resonators stacked one on top of the other. However, performance of this design has been limited by filling factor considerations. A transformer-coupled double-tuned coil where two coil elements are wound concentrically in the same plane with a very high multiple inductance between them has also been disclosed to keep both of the coil elements in close proximity to the sample so as to minimize the filling factor. However, opposing currents in the high-frequency mode introduced losses into such a design. Fitzsimmons et al. thus concluded that no prior art double-tuned surface coil design has been completely free of circuit losses.
Such prior art double-tuned surface coil designs will now be discussed with reference to prior art FIGS. 2-5.
The afore-mentioned double-tuned loop gap resonator probe disclosed by Hyde et al. is shown in prior art FIG. 2. In FIG. 2, inductors L.sub.1 and L.sub.2 are typically positioned vertically on a cylindrical form. Capacitances C.sub.1 and C.sub.2 tune the inductors to the high frequency mode, while capacitance C.sub.3 tunes the inductors L.sub.1 and L.sub.2 to the low frequency mode. In this design, double resonance is achieved from the two loosely coupled LC circuits. In the high-frequency mode, the two coils constitute a counter-rotating current pair. Connections may be made between the two coils to permit them to operate in series resonance for the low-frequency mode. The high-frequency mode efficiency is limited by the magnitude of the counter-rotating current in the second loop while the low-frequency efficiency is limited by the reduction in filling factor due to the fact that the second loop is physically distant from the sample. This design was shown by Fitzsimmons et al. to be relatively efficient in the high-frequency mode but relatively inefficient in the low-frequency mode.
FIG. 3 illustrates a transformer-coupled doubletuned coil of the type disclosed by Fitzsimmons et al. in an article entitled "A Transformer-Coupled Double-Resonant Probe For NMR Imaging and Spectroscopy", Magnetic Resonance in Medicine, Vol. 5, pp. 471-477 (1987). As described by Fitzsimmons et al. and shown in FIG. 3, a transformer-coupled double-tuned coil is an example of an "overcoupled" primary and secondary circuit where two coils L.sub.1 and L.sub.2 are tightly wound in a coaxial fashion to achieve a high mutual inductance or coupling. Such a high degree of coupling produces two resonances where the frequency difference is determined by the magnitude of the mutual coupling and the values of the primary and secondary capacitances C.sub.1 and C.sub.2. As shown in FIG. 3, there need be no direct electrical connection between the primary and the secondary circuit. L.sub.2 and C.sub.2 form a high-frequency circuit, while L.sub.1 and C.sub.1 are primarily responsible for the low-frequency mode. The high-frequency efficiency of this circuit is limited by the large counter-rotating current in the primary loop through inductor L.sub.1 ; however, the low-frequency mode will have current flow in the same direction in both loops and will provide a good filling factor. An additional capacitor between the coils L.sub.1 and L.sub.2 enables the frequencies of both circuits to be matched to the transmission line impedance.
FIG. 4 illustrates a double-tuned trap circuit of the type disclosed by Schnall et al. in an article entitled "A New Double-Tuned Probe For Concurrent .sup.1 H and .sup.31 P NMR", Journal of Magnetic Resonance, Vol. 65, pp. 122-129 (1985). In the double-tuned trap circuit of FIG. 4, the inductor L.sub.sample is the inductor in contact with the sample while the inductor L.sub.trap is positioned on the circuit board along with the other components. Inductor L.sub.trap and capacitance C.sub.trap make up the "trap" circuit which permits tuning the network to the high-frequency mode while the combination of the trap circuit and capacitance C.sub.m allows tuning to the low-frequency mode. Such a design is based on the use of additional inductive and capacitive components placed outside of the main inductor that is coupled to the sample. In other words, as shown in FIG. 4, the trap design incorporates a parallel resonant circuit within a parallel resonant circuit which represents, in the high-frequency mode, an impedance which is predominantly capacitive in phase appearing in series with a large capacitor C.sub.2. High-frequency losses are minimized by maximizing the trap inductance, which essentially causes the two capacitors C.sub.trap and C.sub.2 to be connected in series. In the low-frequency mode, on the other hand, the trap is predominantly inductive in phase. In this mode, losses are minimized by minimizing the trap inductance since this inductance appears in series with the sample inductor. In practice, a small trap inductance is used since it is typically more important to maximize performance in the low-frequency mode.
The transformer-coupled double-tuned coil of FIG. 3 and the double-tuned trap circuit of FIG. 4 have been shown by Fitzsimmons et al. to be very efficient in the low-frequency mode but relatively inefficient in the high-frequency mode. This inefficiency is believed to be in part due to the inability of the coils to be independently adjusted to match the impedance of the transmit/receive circuitry and to be easily tuned without having to separately tune for the effects the coils have on each other. An improved probe design is desired.
FIG. 5 illustrates an improved surface coil design in which the sample coil L.sub.sample is driven by the spectrometer and the inductors L.sub.1 and L.sub.2 are configured perpendicular to each other so that they will not couple to each other, thereby minimizing losses. In this configuration, the impedance of the entire probe can be varied by moving L.sub.d with respect to L.sub.sample and changing the coupling between them. However, the impedance matching at each frequency of such a surface coil design cannot be separately adjusted.
Accordingly, a multiply-tuned probe is desired which may be simultaneously matched to the impedance of the rest of the transmit/receive circuitry at a plurality of resonant frequencies without the aforementioned impedance matching and tuning difficulties. Moreover, it is desired to provide a probe which may be tuned to three or more resonant frequencies while still allowing for easy tuning and high efficiency. The multiply-tuned probe of the invention has been designed to meet these needs.